The present invention relates to a signal processing method for processing a signal having been transmitted via a transmission channel. Also, the present invention relates to a corresponding signal processing device.
In telecommunication systems such as mobile telecommunication systems, for example, data signals are exchanged between a first transceiver and (at least) a second transceiver.
For the subsequent explanations, it is focused on a communication between said first and a specific one of said second transceivers.
Namely, in such a communication, initially transmitted data can be subjected to various external influences resulting in a deterioration of the transmitted data at the receiver side. For example, in a mobile telecommunication system such as the GSM system operating according to TDMA (Time Divisional Multiple Access), data may be subjected to an attenuation in the course of propagation along a transmission path between the first and second transceiver devices (base transceiver station BTS and mobile station MS, for example). Also, depending on the environment in the xe2x80x9cvicinityxe2x80x9d of the transmission path, a phenomenon known as multi-path propagation may occur.
The transmission path as the respective xe2x80x9cchannelxe2x80x9d for the data to be communicated in such cases assumes a channel transfer function which differs from the assumed ideal channel transfer function. Such a channel transfer function is also referred to as channel impulse response.
Therefore, in order to achieve an error-free communication between first and second transceivers, at the receiver side, the received (deteriorated) data signals have to be processed to reconstruct the initially sent signals. This reconstruction is effected by so-called equalizers provided for at a respective receiver side.
In principle, an equalizer processes the received signals having been subjected to a non-ideal channel transfer function, by applying the inverse of the non-ideal channel transfer function/channel impulse response to the received signals.
To this end, an estimation of the channel impulse response of the (non-ideal) transmission channel is required.
Various concepts of channel impulse response estimation have already been proposed in literature, which are based on a statistical evaluation of the received data signals in order to estimate a channel impulse response function.
As an important statistical parameter in mobile telecommunication systems, noise energy is estimated from received data signals. The energy of noise (or energy of estimated interference) is closely related to the variance of the received signals. Hence, an estimate of the variance of the received signals is used as the energy of noise in such systems.
This estimate of the signal variance (noise energy) is used for determining the quality of received signals (e.g. expressed as the Bit Error Rate BER), and is also used in the receiver, for example in connection with the estimation of a channel impulse response function for equalization of the received signals.
Hitherto, various approaches are described in literature, for obtaining an estimate of noise energy/variance as a result of a signal processing method for processing a signal having been transmitted via a transmission channel
Firstly, according to one known approach, the noise energy is calculated based on the difference between a received signal and a result of a convolution of a training sequence (contained in a part of a transmitted signal) and an estimated impulse response. However, performing a convolution operation has a drawback in that it is a cumbersome operation in regard of signal processing power and time consumed. Moreover, information contained in the training sequence has to be extracted from the received signals in order to perform the convolution operation, which additionally imposes a timing/detection problem to the receiver for extracting the training sequence.
Secondly, there are receiver types which estimate the variance (noise energy) according to results obtained by applying the Viterbi algorithm. However, Viterbi algorithm is only optimum as long as the channel impulse response is known, so that an incorrect or only roughly approximated channel impulse response function will lead to incorrect results of the estimated variance.
Thirdly, there exists an approach according to which the signal variance is estimated based on the energy of the received signal and the energy of channel impulse response taps. A respective channel impulse response tap can be considered to correspond to a respective path component of a multi-path propagation channel model. Hence, also for this method, it is either necessary to know the (exact) channel impulse response function in order to have a knowledge of impulse response taps, or to use an approximated, hence, incorrect, multi-path channel model (e.g. restricted to only two or three path model). However, an exact knowledge of the impulse response is rather difficult to obtain, while using an approximated model leads to incorrect estimation results of the variance (noise energy).
From Patent Abstracts of Japan, Vol. 011, No. 020(E-472), Jan. 20, 1978 and JP-A-61189751 (Fujitsu Ltd.), Aug. 23, 1986 a signal processing method for processing a signal having been transmitted via a transmission channel, the method comprising the steps of receiving said signal, analyzing the amplitude of a part of said received signal, and estimating the variance of said signal based on the amplitude of said part, as well as a corresponding processing device is known.
Hence, it is an object of the present invention to provide a signal processing method for processing a signal having been transmitted via a transmission channel, which is free from the above drawbacks inherent to prior known approaches. Furthermore, it is an object of the present invention to provide a corresponding signal processing device for processing a signal having been transmitted via a transmission channel.
According to the present invention, this object is achieved by a signal processing method for processing a signal having been transmitted via a transmission channel, the method comprising the steps of:
receiving (S11) a plurality of samples of said signal;
analyzing (S12) the amplitude of a subset of said plurality of received samples; and
estimating (S13) the variance of the signal based on the amplitude of the subset of said plurality of received samples,
wherein the subset of the plurality of samples is defined as a number (N) of samples within an interval ((x:y)) of the plurality of received signal samples, with x being a first sample of the subset and y being a Nth sample of the subset,
wherein the step of analyzing (S12) the amplitude comprises
a first detecting step (S21) of detecting an absolute maximum (refi) among imaginary parts of the number (N) of signal samples, and
a second detecting step (S22) of detecting an absolute maximum (refq) among real parts of the number (N) of signal samples,
wherein in the first detecting step (S121), the absolute maximum (refi) among imaginary parts is detected according to a relation refi=max(|Qdata(x:y)|), and
in the second detecting step (S122), the absolute maximum (refq) among real parts is detected according to a relation refq=max(|Qdata(x:y)|), wherein Idata represents an imaginary part of a received sample, and Qdata represents a real part of a received sample, and
wherein in the estimating step (S13) the variance (var) is estimated according to a relation                     var        =                  xe2x80x83                ⁢                  A          *          var          ⁢                      xe2x80x83                    ⁢          p                                        =                  xe2x80x83                ⁢                  A          *                      1            /            N                    *                                    ∑                              k                =                1                            N                        ⁢                          xe2x80x83                        ⁢                          (                                                                    (                                          refi                      -                                              "LeftBracketingBar"                                                  Idata                          ⁡                                                      (                                                          x                              ⁢                                                              :                                                            ⁢                              y                                                        )                                                                          "RightBracketingBar"                                                              )                                    2                                +                                                      (                                          refq                      -                                              "LeftBracketingBar"                                                  Qdata                          ⁡                                                      (                                                          x                              ⁢                                                              :                                                            ⁢                              y                                                        )                                                                          "RightBracketingBar"                                                              )                                    2                                            )                                          
where Idata(x:y) and Qdata(x:y), respectively, denote a respective imaginary/real component of an kth sample within the subset of N samples in which x represents the first sample and y represents the Nth sample, and varp represents a preliminary variance to be scaled by a scaling factor A.
Also, the above object is achieved by a signal processing device (30) for processing a signal having been transmitted via a transmission channel (31), the device comprising:
receiving means (32,33) adapted to receive a plurality of samples of the signal;
analyzing means (34, 34a, 34b) adapted to analyze the amplitude of a subset of the plurality of received samples; and
estimating means (35, 35a) adapted to estimate the variance of the signal based on the amplitude of the subset of the plurality of received samples,
wherein the subset of the plurality of samples is defined as a number (N) of samples within an interval ((x:y)) of the plurality of received signal samples, with x being a first sample of the subset and y being a Nth sample of the subset, the subset being buffered in a buffer means (33),
wherein the analyzing means (34) further comprises
a first analyzing element (34a) adapted to detect an absolute maximum (refi) among imaginary parts of the number (N) of signal samples, and
a second analyzing element (34b) adapted to detect an absolute maximum (refq) among real parts of the number (N) of signal samples,
wherein the first analyzing element (34a) is adapted to detect the absolute maximum (refi) among imaginary parts according to a relation refi=max(|data(x:y)|), and the second analyzing element (34b) is adapted to detect the absolute maximum (refq) among real parts according to a relation refq=max(|Qdata(x:y)|), wherein Idata represents an imaginary part of a received sample, and Qdata represents a real part of a received sample, and
wherein the estimation means (35) further comprises calculation elements (35a), the calculation element being configured such that the variance (var) is estimated according to a relation                     var        =                  xe2x80x83                ⁢                  A          *          var          ⁢                      xe2x80x83                    ⁢          p                                        =                  xe2x80x83                ⁢                  A          *                      1            /            N                    *                                    ∑                              k                =                1                            N                        ⁢                          xe2x80x83                        ⁢                          (                                                                    (                                          refi                      -                                              "LeftBracketingBar"                                                  Idata                          ⁡                                                      (                                                          x                              ⁢                                                              :                                                            ⁢                              y                                                        )                                                                          "RightBracketingBar"                                                              )                                    2                                +                                                      (                                          refq                      -                                              "LeftBracketingBar"                                                  Qdata                          ⁡                                                      (                                                          x                              ⁢                                                              :                                                            ⁢                              y                                                        )                                                                          "RightBracketingBar"                                                              )                                    2                                            )                                          
where Idata(x:y) and Qdata(x:y), respectively, denote a respective imaginary/real component of an kth sample within the subset of N samples in which x represents the first sample and y represents the Nth sample, and varp represents a preliminary variance to be scaled by a scaling factor A.
Favorable refinements of the present invention are set out in the dependent claims.
Accordingly, the present invention advantageously provides a very simple method to estimate the signal variance (noise energy). Still further, the present invention can advantageously be used in connection with LMMSE based type of channel impulse response estimation devices and methods as for example proposed in same applicants former patent application PCT/EP 98/07393, where the variance is supplied as a parameter to the estimation device.
Also, with the present invention it is possible to estimate the variance on the basis of the amplitude of the received signal. Consequently, no information about channel impulse response or training sequence is required. Additionally, no xe2x80x9cpreprocessingxe2x80x9d is required in order to obtain a channel impulse response function on the basis of which the variance is subsequently estimated, which reduces the required processing time.
The latter in turn removes the above mentioned timing/detection problems for extracting the training sequence from the received signals.
When applied to a diversity type receiver device, results of experiments conducted by the present inventor show that a ratio between a main branch variance and a diversity branch variance is correct, so that the proposed invention can also be applied to a diversity type receiver.